However, I do think it could be made clearer in the docs that MIT
kerberos is not suitable for use with OpenLDAP.

Well, that err on the other side. There are plenty of situations
where the MIT implementation is indeed suitable, and since it can
be a gigantic hassle to build Heimdal just for slapd when the MIT
libraries are already there, there's a cost on either side of
over-simplifying this issue.